My experience with California smog is that if you keep your car reasonably maintained, it's tough to fail. The maximum allowable values are high enough to never hit on a healthy BMW engine. So unless you have a CEL that you've been ignoring for the last couple years, I think you'll be ok. Every other year only.

If the place is legit, it will go through visual inspections and the sniffer. Every shop will try to "negotiate" the price higher for the test, but if you pay under $40 out the door I think you've done ok.

Here's my last two tests for the 325 for reference on what to expect. Click for a bigger image.